Explosive driven conical shock tube



May 25, 1965 w. s. FILLER EXPLQSIVE DRIVEN CONICAL SHOCK TUBE 2Sheets-Sheet 1 Filed Dec. 2'7. 1960 INVENTOR. WILLIAM s. FILLER I"; a l

I $31 ATTYS- y 1965 w. s. FILLER 3,184,955

EXPLOSIVE DRIVEN CONICAL SHOCK TUBE Filed Dec. 27, 1960 2 Sheets-Sheet 2START\ FSTOP TIMING CIRCUIT AMPLIFIER OSCILLOSCOPE 42 F I (3.3. v

32 27 35 34 K-IO AMPLIFIER 38 I 43 K 2') (I) 3 44 45 (I 0.

TIME

mmvron WILLIAM S. FILLER United States Patent EXPLOSIVE DRIVEN CGNECALSHOCK TUBE William S. Filler, Rockviile, Md, assignor to the UnitedStates of America as represented by the Secretary of the Navy Filed Dec.27, 1960, Ser. Nos 73,794

Claims. (Cl. 73-35} (Granted under Title 35, U.S. Code (1952), see. 266)The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to a device for greatly amplifying the blasteffect of a high explosive charge and more particularly to a testapparatus for generating a spherical blast wave from the explosion of asmall amount of high explosive within a conical shock tube for thesimulation of a spherical shock wave having a characteristic equivalentto the shock wave produced from a much larger quantity of explosivefired in the open.

High explosive detonations and the shock waves they produce in air havebeen the subject of extensive studies for several years. Such studies,however, have been hampered by the difficulty in obtaining experimentalmeasurements of the blast wave and by adverse field conditions underwhich the wave was produced. A further disadvantage is the requirementof unusual facilities to manufacture, handle, and test-fire largeexplosive charges necessary to effect the blast intensity of theexplosion at a selected distance from the source thereof. Variousmethods have been employed heretofore to generate spherical shock Wavesunder convenient laboratory conditions. In one such method a brittlespherical container filled with compressed gas was ruptured. In anothermethod a conical-shaped shock tube having a diaphragm separating highand low pressure sections at the small end thereof has been employed insuch manner that rupture of the diaphragm causes a flow of gas from ahigh pressure chamber at the apex part of the tube which generates ashock wave comprising a sector of a spherical wave. In all of thesemethods which employ compressed gas as the driving means for generatingthe blast, only weak blasts were obtained.

According to the present invention only a small quantity of highexplosive disposed within a firing chamber in communication with thesmall end of a conical shock tube is employed to generate a sphericalshock wave within the tube of greatly amplified character andcorresponding in blast intensity or strength to that of a much largerquantity of high explosive detonated in the free air at a selecteddistance from the point of measurement thereof. The amplification factorof the apparatus varies in an inverse ratio to the angle of the conicalinterior surface of the shock tube.

One of the objects of the present invention is the provision of new andimproved means for greatly amplifying the blast effect of a highexplosive charge at a selected distance therefrom as the charge isfired.

Another of the objects is the provision of new and improved means forgenerating spherical blast waves and amplifying the blast effect thereofat a selected distance from the wave source.

Still another object is the provision of means for generating aspherical blast wave and greatly amplifying the blast effect thereof ata selected distance from the source of the wave and measuring the timeof transit of the wave from the source to the point of measurementthereof.

A still further object is to provide a new and improved test apparatusfor greatly amplifying the explosive effect of a small high explosivecharge disposed therein in which a pressure sensing element arrangedwithin a conical shock tube at a selected distance from the explosivesource is employed to control the recordation of a pressure-time recordof the spherical wave sensed thereby and to effect a control forindicating the precise minute interval of time corresponding to the timeof travel of the pressure wave from the source thereof to the sensingelement.

Various other objects, advantages and improvements will be more readilyapparent upon consideration of the following specification taken inconnection with the accompanying drawings of which:

FIG. 1 is a view partially in section and partially broken away of aconical shock tube employing the instant invention in accordance with apreferred form thereof;

FIG. 2 is a. diagrammatic view of the device of FIG. 1 and a systemsuitable for use therewith;

FIG. 3 is a greatly enlarged view partially in section of a fragmentaryportion of the shock tube and a pressure sensing element secured theretosubstantially flush with the inner surface thereof; and

FIG. 4 illustrates a time-pressure record of the instantaneous value ofthe pressure of the shock wave sensed by the pressure sensing means.

Referring now to the drawings for a more complete understanding of theinvention on which like numerals or references are employed to designatelike parts throughout the several views, there is shown in FIG. 1thereof a shock tube of frusto-conical configuration indicated generallyby the numeral it) and having a flange 11 welded thereto substantiallyas shown. A cylindrical firing block 12 having a conical recess 13therein continuous with the inner conical surface of shock tube 10 isbolted to flange 11 as by the bolts 14.

There is provided within the cylindrical firing block 12 a circular bore15 in communication with the conical recess 13 and a second bore 16coaxial therewith. Disposed within the circular bore 15 is anelectro-responsive detonator 17 having a pair of leads 18 connectedthereto and protruding from bore 16 to establish an electrical firingcircuit to the detonator. The detonator is preferably provided with anon-metallic casing such, for example, as a plastic casing suitable forthe purpose, to prevent interference with the spherical shock wave orpressure sensing apparatus as the result of flying metal fragments asthe detonator is exploded.

There is also provided within the shock tube a hollow tubular support 19secured to the casing of the shock tube by a resilient mounting member21 through which the tubular support extends, substantially as shown.The support 19 is provided with a portion 22 formed at an anglepreferably with respect to the part of the tube protruding through themounting member 21 in such a manner as to be directed toward theexplosive charge which, in the illustrated embodiment, i a detonator.There is also provided a pressure sensing element 23 preferablycomprising a tube of lead zirconate, the dimensions of which arepreferably &1 inch in diameter and the length thereof /42 inch. To thispressure sensing element or gage are secured a pair of conductors 24disposed within the tubular support 19 for establishing an externalelectrical connection thereto. The sensing element 23 is provided with anose portion 25 having a length substantially eight times the diameterof the gage to minimize disturbing effects of the sensing element as aresult of the flow of the gases past the gage. The gage 23 is secured tothe portion 22 of mounting member 21 preferably by a resilient couplingmember 26 which, together with member 21, effectively prevents vibratoryor oscillatory movement of the sensing as a result of vibrationstransmitted longitudinally through the material of which the shock tubeis composed. Spurious pressure indications of pressure received by thesensing element 23 are ens ass thus substantially eliminated. Anarrangement is thereby rovided in which the sensing element ismaintained in a predetermined position within the shock tube at a selected distance from the source of the explosive blast.

The shock tube it is also provided with a plurality of apertures 27therein at different selected distances from the detonator, each ofwhich is adapted to receive a fitting 28 threadedly securedto an annularresilient support 29 composed of non-conducting electrical material andclamped to the shell of the shock tube as by screws 31, a resilientwasher or packing member 32 of annular configuration being disposedbetween the support 29 and the casing of shock tube 10, which togetherwith resilient washers 33 arranged beneath the heads of the screws 31effectively prevent the transmission of longitudinal vibrations throughthe shell of shock tube to the fitting 28. The fittting 28 is preferablyrecessed as at $4 to receive and support a wall-type piezoelectric gage35 in substantial flush relation with respect to the adjacent innerately adjacent thereto when the plates are secured to the tube.

On FIG. 2 is shown a complete system suitable for use with the shocktube device of FIG. 1 with particular reference to a pressure sensingelement supported by tubular support 19 and having the pair ofconductors 24 thereof connected to an amplifier. The output of theamplifier is connected to an oscilloscope substantially as shown and tothe stop terminal of a timing circuit as by conductor 33. The startterminal of the timing circuit is connected as by conductor 39 to thefiring block 12 for the purpose which will be more clearly apparent asthe description proceeds. ground at 41. The oscilloscope is arranged inthe field of vision of a drum type recording camera &2 in such a mannerthat a record is made on a time axis of the image appearing on theoscilloscope corresponding to the pressure wave sensed by the sensinglement 23.

On FIG. 4 is shown on a time axis a pressure wave sensed by the pressuresensing element as the element is enveloped by the pressure wave of ablast received from the detonator. As shown on FIG. 4 the pressure risessuddenly to a maximum value at 43 as the front of the pressure waveenvelops the sensing device. The wave thereafter drops to a zero valueat 44 and thereafter becomes slightly negative until the point 45 of thetrace is reached. This graph is characteristic of a spherical pressurewave by exhibiting an explosive blast pressure wave having a well knowndiscontinuous shock front followed by rapid decay and which returns toambient pressure in about 500 microseconds. A drum camera suitableforthe instant purpose is one which is capable of providing a onemillisecond per inch time resolution of the film.

When firing impulse is applied to the firing leads 18 causing operationof detonator 17, the resulting expiosion of the detonator'is accompaniedby an ionization of gas which causes a negative electron flow away fromthe ionized gas region. This negative flow causes a negative startimpulse to be applied to conductor 39 sufficient to set the timingcircuit into operation. 'When the pressure wave sensed by the sensingelement reaches point 43 of the graph'of FIG. 4, a stop impulse isapplied by way of conductor 38 to the timing circuit thereby givingan'indication of the time of travel of the explosive wave from thedetonator to the sensing element.

Itxhas been found by actualtest that when the angle of the shock tube is22 degrees, the amplification thereof The timing circuit is connected tois 62. Thus, when a high explosive charge of 0.5 gram, for example, isemployed in the firing block 12 as a source of lngh pressure wave blast,the strength of the blast measured at the sensing element is equivalentto the blast received in free air from 31 grams of explosive at the samedistance from the source of the explosive blast.

A suitable method of calibrating the blast gages employed with thepresent invention is to expose them to shock waves whose peak pressureswere accurately known. This was done lay means of free-field firings ofa standard explosive. Sensitivity of the gage could be determined fromrecordings of the shock wave obtained with a particular gage. 7

Quantitative evaluation of the performance of the shock tube may beobtained by measuring the peak pressure (P), positive impulse (I),positive duration (T) of the shock wave (FIG. 4) that was recorded atseveral distances from the apex of the cone. The positive impulse of theshock Wave is defined by the expression were p pressure and t =tinie.

This integral is evaluated numerically by determining the area under thepressure-time curve such, for example, as shown on FIG. 4.

The pressure sensing arrangement .of FIG. 3, it will be noted, comprisesa pressure sensing element or pres sure gage connected as by theconductors 37to an amplifier and thence to an oscilloscope generally inthe manner of FIG. 2, the timing circuit being connected by conductor38, if desired, totheoutput of the amplifier in the manner of FIG. 2.Furthermore, whereas on FIG. 2 is shown but a single sensing element 23in substantial alignment with the axis of the cone, additional sensingelements 23 may be provided, each on its support 19 at differentselected distances from the explosive charge and not necessarily on theaxis of the cone but each pointing in the direction of the source of theexplosive blast. Also, if desired, additional pressure gages 35, FIG. 3,may be employed, if desired, or a combination of sensing elements 23 and35 may be employed to obtain a desired result. 7

The angle of the cone shaped shock tube 10 may be other than the22-degree angle illustrated. For example, a cone having a two-degreeangle possesses a theoretical amplification factor of 10,000. Actualtest of a two-degree cone indicated an actual amplification of 3,000which, although corresponding to an etficiency of about 30% with thiscone, a truly spectacular degree of amplification was made possible byits use.

While the invention has been'described with reference to two embodimentsthereof which give satisfactory results, it will be obvious to thoseskilled in the art to which the invention appertains, afterunderstanding the invention, that the same is susceptible of additionalembodiments, modifications and variations thereof without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Test apparatus for greatly amplifying the blast effect of a smallhigh explosive charge comprising a rigid frustoconical tube foramplifying a shock wave, a firing block having a firing chamberthereinin communication interiorly withsaid tube and disposed inabutting relationship with the small end thereof, and a small solid highexplosive detonating charge disposed within said chamber for generatinga shock wave upon detonation and having a pair'of firing leads connectedthereto and extending through said'block, and a piezo-electrical airblast pressure gage disposed within said tube at a selected distancefrom said charge for sensing the blast effect from said high explosivedetonating charge.

2. Test apparatus according to claim 1 including means connected to-saidgage for amplifying and recording the 5 signal therefrom as a blast wavefrom said charge envelopes the gage.

3. Test apparatus according to claim 2 including a counter timer havingmeans for indicating precise minute intervals of time, meansinterconnecting said counter timer to said block in a manner to apply anelectrical start impulse thereto at the instant the charge is fired, andmeans interconnecting said counter timer to said recorder in a manner toapply an electrical stop impulse thereto as a signal from the blast gageis received.

4. Test apparatus according to claim 1 in which said air blast pressuregage comprises a pressure sensing element disposed within said tubesubstantially along the axis thereof.

5. Test apparatus according to claim 1 in which said air blast pressuregage is directed toward said charge.

6. Test apparatus according to claim 1 in which said air blast pressuregage is secured to said tube in substantial alignment with a portion ofthe inner surface thereof.

7. Test apparatus according to claim 6 in which means are provided foryieldably securing the gage to the tube in a manner to prevent vibratorymotion thereof in response to longitudinal vibrations of the tube as thecharge is fired.

8. A device for greatly amplifying the blast effect of a condensed solidhigh explosive detonating charge which comprising a shock tube means offrusto-conical configuration for amplifying the shock wave, a firingblock having a recess therein secured to and communicating with theinterior of said tube and disposed in abutting relation with the smallend thereof, a high solid explosive detonating charge for generating ashock wave upon detonation disposed within said recess and having a pairof leads for establishing a firing circuit thereto, and means disposedwithin said tube for sensing the instantaneous value of the pressure ofthe shock wave caused by the firing of said explosive charge and sensedby said sensing means.

9. A device according to claim 8 including means connected to saidfiring block and said sensing means for rendering manifest the time oftransit of the pressure wave from the charge upon explosion thereof tosaid sensing means.

10. A device according to claim 9 including means coupled to saidsensing means for indicating the pressure of the pressure wave at theinstant of envelopment with the sensing means.

References Cited by the Examiner UNITED STATES PATENTS 2,799,788 7/57Fitzgerald -35 X 2,865,463 12/58 Itria 18153 OTHER REFERENCES ARSJournal, vol. 29, May 1959, Article by Nagamatsu et al., pages 332340.

ISA Journal, vol. 5, November 1958, Article by Bershader, pages 627l.

ISA Journal, vol. 7, August 1960, Article by Harris et al., pages 62-66.

Rev. Sci. Instr., vol. 29, February 1958, Article by Knight, pages 174,175.

RICHARD C. QUEISSER, Primary Examiner.

ROBERT L. EVANS, DAVID SCHONBERG,

Examiners.

1. TEST APPARATUS FOR GREATLY AMPLIFYING THE BLAST EFFECT OF A SMALLHIGH EXPLOSIVE CHARGE COMPRISING A RIGID FRUSTOCONICAL TUBE FORAMPLIFYING A SHOCK WAVE, A FIRING BLOCK HAVING A FIRING CHAMBER THEREININ COMMUNICATION INTERIORLY WITH SAID TUBE AND DISPOSED IN ABUTTINGRELATIONSHIP WITH THE SMALL END THEREOF, AND A SMALL SOLID HIGHEXPLOSIVE DETONATING CHARGE DISPOSED WITHIN SAID CHAMBER FOR GENERATINGA SHOCK WAVE UPON DETONATION AND HAVING A PAIR OF FIRING LEADS CONNECTEDTHERETO AND EXTENDING THROUGH SAID BLOCK, AND A PIEZO-ELECTRICAL AIRBLAST PRESSURE GAGE DISPOSED WITHIN SAID TUBE AT A SELECTED DISTANCEFROM SAID CHARGE FOR SENSING THE BLAST EFFECT FROM SAID HIGH EXPLOSIVEDETONATING CHARGE.